Expansion tank requirements?

Are there any requirements on installing an expansion tank? Like distance between fittings or between anything?
I had a know-it-all father-in-law saying this and saying that about everything but he was adamant that the expansion tank had to be 18" from the 90 degree. I let him go on but I wanted to check anyway.
These pictures show the expansion tank, clearly not 18".

Thanks for the help.

Below’s comment copied from: https://www.flexconind.com/pdf/wh-whv_install.pdf


  1. This tank must be installed according to your local plumbing code. It should be
    at least 18” from the cold water inlet to the water heater…*

Thanks Marcel, Good information.
The expansion tank is on the output line of the furnace.
Does that make a difference?

It looks like an Amtrol and they do not have that requirement and the unit manufactured by Watts does not have that requirement either. They just need to be installed on the cold water side between the water heater and backflow device.

Thank you Stephen,
The tank is in the middle of the hot water exiting the boiler to the start of the hot water heat loop.